Optical fibers are key components in modern telecommunications. These key optical fibers are thin strands of glass capable for transmitting an optical signal containing a large amount of information over long distances with very low loss. In essence, an optical fiber is a small diameter waveguide characterized by a core with a first index of refraction surrounded by a cladding having a second, lower index of refraction. Typical optical fibers are made of high purity silica with minor concentrations of dopants to control the index of refraction.
As well known, single fibers can carry multiple packets of data that are multiplexed on the fiber either by time division where different slots of time are allocated to different packets or by wave division multiplexing where different wavelengths are allocated for different data. Modulators and switches that perform the important function of adding information content to optical signals in optical communications systems have been fabricated in the past using several different techniques. The most popular ones are based on the electrooptic effect in lithium niobate substrates. Other means of achieving the same are based on gallium arsenide technology or use indium as an active dopant in the planar waveguides.
All the techniques have been implemented on planar structures and hence suffer from two common problems; 1) interconnection losses to optical fibers are high, and 2) optical properties of the two polarizations are very different. Therefore, there is a need for an all-fiber modulator, which due to its circular symmetry and compatibility with the information transmission medium (fiber), sidesteps both the aforementioned problems in existing technologies.
In other applications of optical fiber communication systems, it is desirable to remove the light carrying the optical data or "tap" it out from the core of the fiber to either filter out certain wavelengths of light or to read certain signals passing through the optical fiber to insure power or data quality within the optical fiber. A switchable tap is useful when the tapped light is needed on an intermittent basis.
Typically, the light removal has been accomplished by the use of short periodic gratings written into the fiber's core. Other types of devices used to remove light from the optical fiber includes the use of optical tapping couplers, such as T-type couplers or regenerative couplers, or of optical converters connected to the destination end of the optical fiber.
Such systems present a number of difficulties. For example, most short period gratings typically have periodicities of 1/2 microns. When such short periodicities are present, the light carrying the optical signal gets reflected back in the direction from which it came. When the light gets reflected back in this manner, it is more difficult to couple the light from the core. Moreover, because the short periodic gratings reflect the light backward from the direction of propagation, they have by their nature a low degree of sensitivity to external modulation and are thus not effective as an optical switching mechanism, and as such they are typically used for tuning purposes only. The mechanical tapping coupler devices, on the other hand, are also not desirable because additional switching devices or converters must be used in conjunction with the coupling devices to achieve the switching capability and the switching must be done at either the optical signal's point of origination or termination, thereby eliminating any advantages associated with tapping into the core at a midpoint between the optical fiber's ends. Additionally, in many of these devices it is necessary to grind into the cladding by about 40 to 50 microns in order to assure a high quality data readout. In such instances, the structure of the optical fiber can be compromised, which can affect the quality of optical data within the core.
Therefore, there is a need in the art for an optical switching system that is simple in design, versatile in the point of application to the optical fiber and that has a high degree of sensitivity, thereby allowing the switching function to be accomplished as great speeds. The optical switching system of the present invention addresses these needs.